In a high-frequency circuit unit of a mobile terminal such as cellular phone and smartphone, a transmitter circuit and a receiver circuit are selectively connected to a common antenna through a switch circuit for high-frequency signals. Conventionally, a HEMT (High Electron Mobility Transistor) using a compound semiconductor has been used as a switch element of such a high-frequency signal switch circuit. However, due to the demands for price reduction and downsizing in recent years, it is suggested to use a MOSFET (Metal Oxide Semiconductor Field Effect Transistor) formed on a silicon substrate, instead of the HEMT.
Note that a general MOSFET formed on a silicon substrate has such problems that the parasitic capacitance between the source or drain electrode and the silicon substrate is large, and that power loss of high-frequency signals is large since silicon is a semiconductor. Thus, a technique for forming the high-frequency switch circuit on an SOI (Silicon On Insulator) substrate has been proposed.
Recent mobile terminals operate in multimode and multiband, and the number of ports required for a high-frequency switch has been increased to over ten. The increase in the number of ports inevitably increases the number of bits of a signal required to control the connection state of the switch. For example, an SP10T switch for switching the connections between an antenna terminal and ten RF terminals requires ten connection states, and therefore the number of bits required for the control signal is four. In order to input a 4-bit control signal as parallel signals, four terminals are naturally required. This input manner is hereinafter referred to as a parallel input manner. On the other hand, it is also possible to input a serial data signal in synchronization with a clock signal. This input manner is hereinafter referred to as a serial input manner. The serial input manner has an advantage that only one data input terminal is required even when the number of ports increases. Therefore, demand for the serial input manner has been increased recently although the parallel input manner is mainly used in the conventional high-frequency switch.
Basically, either of the input manners is exclusively used in a high-frequency switch IC, but it is desirable to electrically switch the input manners in order to improve the versatility of the high-frequency switch IC. However, in order to realize this configuration, input terminals based on the respective input manners and terminals for electrically switching the input manners are required, which leads to a problem that the number of pads increase correspondingly.